This is because when you remove the user, their account is automatically disabled. We do this to prevent a removed user from continuing to access diagrams that might have been shared with them and contain sensitive organization information. In order to add them back to the team plan, you will have to contact our support desk to reactivate the user's account.

This could be slightly confusing to some of our users. However, we plan to allow the full control over adding and removing a user in a future revamp of the team management features.

Hope this article addresses your question on adding members back to the team account.